Exhaled breath diagnostics as a clinical method is reaching more and more acceptance in a variety of different diseases. To ensure reproducible measurement results, exhaled breath from patients should conform to certain protocols dictating the physical parameters (e.g. flow rate, pressure, temperature etc.) under which the test should be made. Thus it is sought to perform measurements under substantially constant flow of exhaled breath, despite variations in applied pressure.
In WO 2006/080885 A1 a constant flow regulator device for maintaining a constant flow of fluid is disclosed. The device comprises an inlet duct for incoming fluid, a housing, and a movable partition facing the inlet duct and being subjected to an elastic force. A fluid passage of variable cross section area is formed between the inlet duct and the movable partition. The housing and movable partition form an inner compartment in fluid communication with the inlet duct for establishing a fluid pressure inside the inner compartment approximately equal to the fluid pressure in the inlet duct. The size of the movable partition is significantly greater than the size of the inlet duct such that, in use, the partition is moved towards the inlet duct against the elastic force when the fluid pressure in the inlet duct increases to reduce said fluid passage cross section area, and vice versa, thereby maintaining constant fluid flow.
WO 9505208 discloses a device for the administration of an inhalation medicament, including a body defining a through-going air pathway having a longitudinal axis, an air inlet, an air outlet forming a mouthpiece, means for dispensing medicament into the pathway and air flow regulating means, characterised in that the air flow regulating means includes a movable obstructing means adapted to reduce the cross-sectional area of the pathway at a location between the air inlet and the means for dispensing medicament, and biasing means, whereby the obstructing means is biased into a first resting position in which the cross-sectional area of the pathway is minimum and is adapted to move against the bias of the biasing means to a second position in which the cross-sectional area of the pathway is maximum in response to a pressure fall at the mouthpiece caused by inhalation and is adapted to move further to a third position in which the cross-sectional area of the pathway is less than maximum in response to a greater pressure fall at the mouthpiece caused by inhalation
WO 2008016698 discloses a fluid flow regulating mechanism designed particularly for use in connection with aerosolized drug delivery devices is disclosed. The mechanism includes a housing, planar elastic element, regulating element and positioning component. The elements of the device are configured in a manner such that a flow channel through the housing is opened or closed depending on the flow rate of fluid through a flow channel.
WO 9214199 discloses flow regulating device placed in a fluid path to passively compensate the fluctuations in the fluid pressure by deflection or displacement.
The current trend towards handheld point-of-care devices has brought a need for miniaturized flow handling systems. The previously disclosed device requires a significant size of the partition. It is therefore desired to find alternative solutions to the problem of regulating a fluid flow that may be suitable for miniaturisation, and that still may be used to regulate the relatively large flows of exhaled breath. For example, in asthma monitoring (fractional exhaled NO, FENO), regulatory guidelines dictates that measurements of nitric oxide concentration should be made at an exhaled flow rate of 50±5 ml/s (“ATS/ERS recommendations for standardized procedures for the online and offline measurement of exhaled lower respiratory oxide and nasal nitric oxide”, 2005. Am. J. Respir. Crit. Care. Med. 2005; 171: 912-930).